The disclosure of Japanese Patent Application No. 2002-114850 filed on Apr. 17, 2002 including the specification, drawings and abstract are incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to control of an automatic transmission mounted in a vehicle, and in particular, to control in the case where, during shifting towards a gear stage in which an engine brake friction element is engaged, a command is output that releases the engine brake friction element thereof.
2. Description of the Related Art
An automatic transmission mounted in a vehicle is structured by the combination of a torque converter to which an engine output is input, and a speed change gear mechanism driven by an output of the torque converter. The gear stage automatically shifts toward a predetermined gear stage in response to the demands of a driver or a driving state due to a switching of transmission path of the speed gear mechanism by means of selective engagement and release of a plurality of friction elements such as a clutch and a brake. In such an automatic transmission, an engine brake friction element, in addition to a friction element for changing speed, is provided. Normally, this engine brake friction element only transmits power during driving, and operates an engine brake during deceleration at a predetermined gear stage such as first range or second range by engaging it.
In such an automatic transmission, sometimes the gear stage is fixed at a predetermined gear stage (for example, a downshift from the current gear stage) in each range according to the operation of the driver. As operations for this purpose, the driver may incline a shift knob of the automatic transmission to the downshift side or turns on a sports mode switch, and in the case of a so-called sequential manual transmission, operate a change lever to the minus side or operate a manual switch provided in the steering wheel portion to the minus side. In the following description, these will be collectively described as operating a manual switch. For example, when the manual switch is turned on while traveling in the fourth speed of the D range, the gear stage is downshifted to the third speed and fixed. In addition, when the manual switch is turned on while traveling in the third speed of the S range, the gear stage is downshifted to the second speed and fixed. Also, the engine brake friction element is engaged and released by the operation of this manual switch in accordance with the speed change operation, thereby switching the actuation and release of the engine brake.
In such an automatic transmission, there are cases when multiple speed changes are performed through the operation of the manual switch, sudden changes in the driving state, or the like. Specifically, there are cases such as a command to shift to a second gear stage based upon a second speed change command is output while a speed change operation toward a first gear stage is being performed based upon a first speed change command, and the like. A control device for an automatic transmission that solves problems generated during such multiple speed changes is disclosed in Japanese Patent Laid-Open Publication No. 9-89092.
The automatic transmission control device disclosed in the publication controls an automatic transmission including an engine brake friction element that operates an engine brake during engagement. The control device includes a detecting circuit and a control circuit. The detecting circuit detects that multiple shift commands have been output that release the engine brake friction element during a speed change operation to a gear stage based upon a first speed change command in which the engine brake friction element is engaged and established. Also, in response to the multiple shift commands detected by the detecting circuit, the control circuit controls the automatic transmission such that the speed change operation based upon the second speed change command is not started until after the speed change operation based upon the first speed change command is finished. In addition, the control circuit controls the automatic transmission such that release of the engine brake friction element is forbidden until the speed change operation based upon the first speed change command is finished.
According to the automatic transmission control device disclosed in the publication, when a second speed change command (for example, shifting from first speed to second speed)is output during a speed change operation based upon a first speed change command (for example, shifting from second speed to first speed), the speed change operation based upon the second speed change command is started after the speed change operation based upon the first speed change command is finished, and release of the engaged first speed engine brake friction element is forbidden until the speed change operation based upon the first speed change command is finished. Therefore, according to the control device, once the speed change operation based upon the first speed change command is finished (engagement of the first speed engine brake friction element is complete), the second speed change operation is started. As a result, the first speed engine brake friction element is not released during engaging, and it is possible to reduce shift shock from the sudden inoperativeness of the first speed engine brake friction element.
Further, control different from that in the publication, which aborts the first speed change operation and moves on to the second speed change operation can also be considered. It is assumed that an engine brake friction element (B-4) generating a first speed engine brake and an engine brake friction element (B-2) generating a second speed engine brake are connected to a linear solenoid valve (SL2) via a shift valve. FIG. 6 shows a timing chart regarding a speed change operation from first speed to second speed in such a connected state. As shown in FIG. 6, after the speed change command in this control, the engine brake friction element (B-4) that generates the first speed engine brake is engaged, thereby generating the first speed engine brake. In addition, a timing chart is shown in FIG. 7 regarding a speed change operation in the case where a speed change from first speed to second speed is required during this speed change operation from second speed to first speed. As shown in FIG. 7, after the speed change command from second speed to first speed, in this control, the engine brake friction element (B-4) that generates the first speed engine brake is engaged, and that engagement is aborted while the first speed engine brake is being generated, at which time the linear solenoid (SL2) executes control engaging the engine brake friction element (B-2) that generates the second speed engine brake. Therefore, the engine brake friction element (B-2) that generates the second speed engine brake is engaged, thereby generating the second speed engine brake, while the friction element (B-4) generating the first speed engine brake is released, allowing rapid shifting from first speed to second speed.
However, according to the automatic transmission control device disclosed in the publication, the speed change operation based upon the second speed change command would have to wait until the speed change operation based upon the first speed change command is finished. Therefore, response to the shift demand from the driver is delayed.
Further, a control as shown in FIGS. 6 and 7, different from that in the publication, which aborts the first speed change operation and shifts to the second speed change operation can also be considered. By controlling the automatic transmission in such a manner, responsiveness to shift demand from the driver is improved. However, since the first speed engine brake friction element is released while it is being engaged, a shift shock is generated by the sudden inoperativeness of the first speed engine brake friction element.
It is an object of the invention to provide an automatic transmission control device and a control method which solve the above-mentioned problems and allows excellent response to shift demands from a driver of a vehicle with little shift shock in the automatic transmission during multiple speed changes.
The automatic transmission control device according to an aspect of the invention has an automatic transmission provided with a first gear stage including a plurality of engine brake friction elements operating engine brakes during engagement, and connected to a first shift valve in which a first engine brake friction element among the plurality of engine brake friction elements is engaged and established, and a second gear stage connected to the first shift valve in which a second engine brake friction element among the plurality of engine brake friction elements is engaged and established; a linear solenoid valve connected to the first shift valve; a detecting portion that detects shift demands to the first gear stage or the second gear stage; and a control portion engaging and releasing the first engine brake friction element and the second engine brake friction element through the linear solenoid valve. In addition, the control portion includes a valve control portion that controls the solenoid valve such that a time reduction rate of an output oil pressure from the linear solenoid valve to the shift valve becomes equal to or less than a predetermined rate in response to the detection of a shift demand for a shift from the second gear stage to the first gear stage, and a command portion for commanding the automatic transmission with a shift command from the second gear stage to the first gear stage in response to the passage of a predetermined time since the detection of the shift demand from the second gear stage to the first gear stage.